Secondary batteries, which typically have a large number of battery cells connected in series and/or in parallel, in hybrid and electric vehicles. Electric drive devices of hybrid and electric vehicles are supplied with electrical energy by the secondary batteries. The battery cells can, for example, be based on a lithium-ion technology or on a nickel-metal hydride technology.
In order to be able to optimize the service life of battery cells and consequently of a secondary battery, it is necessary to know at any time the momentary operating state of the battery cells or respectively the secondary battery. A battery management system (BMS) is typically used for testing and regulating a secondary battery, said BMS ensuring a service life of the secondary battery and the battery cells thereof to be as long as possible besides monitoring the safety of said battery.
Besides comprising sensors, which can detect the electrical cell voltage generated by each individual battery cell, the total battery current and a battery temperature, a battery management system comprises a safety function which prevents a maximum admissible electric cell voltage from being exceeded in order to avoid damage to the battery cell or respectively to the secondary battery.
A possible deep discharge of the battery cells can also be counteracted by means of a battery management system. This is advantageous because a discharge of a battery cell below a certain charging threshold can promote adverse chemical processes in a battery cell, which can lead to an irreversible degradation of the battery cells. In order to counteract the aforementioned deep discharge of battery cells, fixed safety limits for the state of charge (SOC) of the battery cells of a secondary battery are predefined, which, however, can be exceeded or undershot by individual battery cells due to various conditions, such as, for example, the irregular ageing behavior of individual battery cells and different capacitive properties and energy densities of individual battery cells, during regular activation of a secondary battery.
According to the current prior art, variants of the so-called “cell balancing” for compensating cell voltage differences do in fact exist; however, even the most effective method of the so-called “autonomous cell balancing” is only initiated after an extended non-operative state of a vehicle equipped with a secondary battery. A “cell balancing” does not take place during the operation of a vehicle.
The European Patent Office patent EP 1 814 206 A1 relates to a device for balancing a multiplicity of at least two battery cells of a multiple cell battery, comprising a multiple cell battery and a battery management system with a balancing circuit. The individual battery cells are connected to a battery monitoring system, wherein the battery management system measures each individual electrical cell voltage, the battery temperature and the electrical current. The battery monitoring system can detect the lowest electrical cell voltage. In addition, the battery monitoring system can detect a number of cells, the electrical cell voltage of which is higher than a certain maximum admissible electrical voltage, wherein these cells are balanced until the electrical voltages of these cells have decreased to an acceptable lower value. The battery management system is active when the battery is being charged or discharged, wherein the threshold values vary with the state of the battery.